FIG. 17 shows the main part of an inker (ink supply device) in a printing unit of each color of a web offset printing press. Referring to FIG. 17, reference numeral 1 denotes an ink fountain; 2, ink stored in the ink fountain 1; 3, an ink fountain roller; 4 (4-1 to 4-n), a plurality of ink fountain keys juxtaposed in the axial direction of the ink fountain roller 3; 5, an ink ductor roller; 6, an ink roller group; 7, a printing plate; 8, a plate cylinder on which the printing plate 7 is mounted; 9, a blanket cylinder; and 10, an impression cylinder. A blanket 91 is mounted on the blanket cylinder 9.
This ink supply device supplies the ink 2 in the ink fountain 1 to the ink fountain roller 3 by adjusting the opening ratios of the ink fountain keys 4-1 to 4-n, and supplies, via the ink roller group 6, the ink supplied to the ink fountain roller 3 to the printing plate 7 by the feed operation of the ink ductor roller 5.
An image is printed on the printing plate 7. The ink supplied to the printing plate 7 is received by the blanket 91 on the blanket cylinder 9. The ink received by the blanket 91 is transferred to printing paper (target printing material) 11 conveyed between the blanket cylinder 9 and the impression cylinder 10.
Note that ink form rollers 6-1 to 6-4 in contact with the printing plate 7 are provided at the end of the ink flow path of the ink roller group 6. Together with the ink supplied via the ink form rollers 6-1 to 6-4, dampening water stored in a fountain pan 13 is supplied to the printing plate 7 via a dampening form roller 12.
In this ink supply device, when a print job is switched, that is, when the printing plate 7 of a preceding print job is exchanged with a printing plate 7′ of the next print job, the opening ratios of the ink fountain keys 4-1 to 4-n and the rotation amount of the ink fountain roller 3 are changed to values corresponding to the image of the printing plate 7′ of the next print job, and the ink 2 in the ink fountain 1 is supplied to the exchanged printing plate 7′ via the ink roller group 6. In this case, test printing is performed before final printing, and the ink supply amount is adjusted, thereby obtaining a satisfactory color tone. A desired ink film thickness distribution (the gradient of an ink film thickness) is thus formed on the ink roller group 6, the plate cylinder 8, and the blanket cylinder 9.
However, in this ink supply device, when exchanging the printing plate 7 with the printing plate 7′ and executing the next print job, the ink film thickness distribution for the printing plate 7 of the preceding print job still remains on the ink roller group 6. In this case, the ink film thickness distribution for the printing plate 7 of the preceding print job needs to be gradually changed to the ink film thickness distribution for the printing plate 7′ of the next print job. Excessive ink supply amount adjustment and test printing are needed until a satisfactory color tone is obtained. This poses problems such as an increase in the preparation time before printing“, an increase in working load”, “a waste of printing materials”, “a decrease in production efficiency”, and “an increase in cost”.
Hence, aiming at decreasing the numbers of times of ink supply amount adjustment and test printing until a satisfactory color tone is obtained, “ink film thickness control methods” described in patent literatures 1 and 2 have been proposed.
[Patent Literature 1 (Ink Decrease+Pre-Inking 2)]
In the ink film thickness control method described in patent literature 1, when switching a print job, the feed operation of the ink ductor roller 5 is turned off. In a state in which the printing plate 7 of the preceding print job is kept mounted, the printing press is operated to print a predetermined number of sheets (blank sheet printing). The ink in the ink supply device is thus decreased (ink decrease), and a minimum ink film thickness distribution Ma (see FIG. 18A) that is needed during printing and becomes thinner from the upstream to the downstream, that is, the ink film thickness distribution Ma corresponding to a portion of the printing plate 7 without any image is left on the ink roller group 6 (ink removing).
Next, the opening ratios of the ink fountain keys 4-1 to 4-n, the rotation amount of the ink fountain roller 3, and the like are set to values corresponding to the image of the printing plate 7′ of the next print job. Then, in a state in which the ink form rollers 6-1 to 6-4 have been thrown off, the printing press is operated to cause the ink ductor roller 5 to perform the feed operation a predetermined number of times, thereby superimposing an ink film thickness distribution Mb (see FIG. 18B) corresponding to the image of the printing plate 7′ of the next print job on the minimum ink film thickness distribution Ma remaining on the ink roller group 6 and needed during printing (pre-inking 2).
[Patent Literature 2 (Ink return to fountain+Pre-Inking 1)]
In the ink film thickness control method described in patent literature 2, when switching a print job, the opening amounts of the ink fountain keys 4-1 to 4-n are set to zero. In this state, the ink ductor roller 5 is caused to perform the feed operation a predetermined number of times, thereby wholly returning the ink on the ink roller group 6 to the ink fountain 1 (ink return to fountain). A state in which each roller in the ink roller group 6 does not hold ink is thus attained.
Next, the opening ratios of the ink fountain keys 4-1 to 4-n are set to a predetermined opening ratio (for example, 50%). In addition, the rotation amount of the ink fountain roller 3 is set to a predetermined amount (for example, 50%). Then, the ink ductor roller 5 is caused to perform the feed operation a predetermined number of times, thereby forming the minimum ink film thickness distribution Ma (see FIG. 18A) needed during printing on the ink roller group 6 (the first step of pre-inking 1).
The opening ratios of the ink fountain keys 4-1 to 4-n, the rotation amount of the ink fountain roller 3, and the like are set to values corresponding to the image of the printing plate 7′ of the next print job. Then, in a state in which the ink form rollers 6-1 to 6-4 have been thrown off, the printing press is operated to cause the ink ductor roller 5 to perform the feed operation a predetermined number of times, thereby superimposing the ink film thickness distribution Mb (see FIG. 18B) corresponding to the image of the printing plate 7′ of the next print job on the minimum ink film thickness distribution Ma formed on the ink roller group 6 and needed during printing (the second step of pre-inking 1).
In the printing press including such an ink supply device, normally, upon switching from the preceding print job to the next print job, ink for the preceding print job remains on the blanket on the blanket cylinder. Hence, to remove the ink, the blanket is cleaned by a blanket cleaning device. The blanket cleaning device, for example, presses a cleaning cloth against the blanket mounted on the blanket cylinder while making the cleaning cloth in a tense state intermittently travel between a supply shaft and a takeup shaft, and sprays a solvent to the cleaning cloth to clean the blanket while rotating the blanket cylinder (for example, see patent literature 3).
For this reason, even if an ink film thickness distribution corresponding to the image of the printing plate of the next print job is formed on the ink roller group by the above-described ink film thickness control method, and after that, the ink form rollers are thrown on, and printing is started, the ink hardly adheres to the blanket because of the influence of the solvent remaining on the blanket, and lightly printed products are produced, resulting in a waste of printing materials.